Semiconductor chips are generally formed with aluminum metallization. The boards upon which they are mounted, however, generally have conductive paths screened on them which are essentially made of gold. When a chip needs to be connected to a substrate on the board, it is necessary to have an aluminum/gold interface. When this happens, the aluminum and the gold react with each other, especially at higher temperatures. This causes two problems. First, the resistance of the junction becomes excessively great. Second, the mechanical strength of the junction becomes excessively small, allowing the wire to work loose and cause an open circuit under forces which must be anticipated in its operating environment, and which would not cause such an open circuit if the junction were stronger.
Two possible solutions to this problem are to make an entirely aluminum apparatus, or an entirely gold apparatus. The former is not satisfactory, since the aluminum tracings on the board react with moisture and other environmental conditions, and rapidly attain an excessively higher resistance. The latter is equally unsatisfactory, if for no other reason than excessive cost.